Quick Facts
Origin: Posterior surface of distal one third of ulna and adjacent interosseous membrane of forearm.
Insertion: Extensor expansion of index finger.
Action: Extends index finger.
Innervation: Posterior antebrachial interosseous nerve (C7-C8).
Arterial Supply: Anterior and posterior interosseous arteries.
Related parts of the anatomy
Origin
The extensor indicis muscle originates from the:
- posterior surface of distal one third of the body of ulna, which is located inferior to the origin site of the extensor pollicis longus muscle;
- adjacent interosseous membrane of forearm.
Insertion
The fibers of the extensor indicis muscle travel inferolaterally to the hand and insert, via a thin tendon, onto the extensor expansion of the tendon of extensor digitorum muscle that goes to the index finger.
Key Features & Anatomical Relations
The extensor indicis muscle is one of the muscles of the deep part of the posterior compartment of the forearm. It is a thin, fusiform type of skeletal muscle. At the wrist, its tendon travels deep to the extensor retinaculum of hand, where it passes through the common tendinous sheath of extensor digitorum and extensor indicis muscles. Within the hand, the tendon then travels inferolaterally to its insertion site.
The extensor indicis muscle is located:
- anterior (deep) to the extensor digitorum muscle and the posterior interosseous artery;
- posterior (superficial) to the distal ends of the ulna, radius, and adjacent interosseous membrane of forearm;
- medial to the extensor pollicis longus muscle.
Actions & Testing
The extensor indicis muscle is involved in multiple actions:
- extends the distal phalanx of index finger at its distal interphalangeal joint;
- extends the middle phalanx of index finger at its proximal interphalangeal joint;
- extends the proximal phalanx of index finger at its metacarpophalangeal joint;
- assists in extension of the hand at the radiocarpal (wrist) joint.
This muscle can contract with the extensor digitorum during extension of all four fingers, or contract alone to fully extend the index finger while the other fingers remain in their fully flexed positions.
The extensor indicis muscle can be tested by extending the index finger while the other three fingers are held in the flexed position. This position of the other three fingers prevents the extensor digitorum muscle from contracting (Standring, 2016).
References
Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41st edn.: Elsevier Limited.
